18" x 26" or 26" x 18" Cube Core Eggcrate Return Air Filter Grille for 1" Filter 18" Diameter + Product Details Size –

+ Product Details Size – 16" x 18" or 18" x 16" is the size for the duct opening measurement only

+ Product Details Perfect Fit & Easy Sizing - Designed for a 12" x 36" or 36" x 12" duct opening (measured at the opening)

+ Product Details Perfect Fit & Easy Sizing - Designed for a 8" x 32" or 32" x 8" duct opening (measured at the opening)

+ Product Details Size - 26"w X 10"h is the size for the duct opening measurement only

18" x 26" or 26" x 18" Cube Core Eggcrate Return Air Filter Grille for 1" Filter 18" Diameter + Product Details Size –+ Product Details Perfect Fit & Easy Sizing Designed for a 18" x 26" OR 26" x 18" duct opening (measured at the opening), with actual outer dimensions of 20. 5" W x 28. 5" H. Please check the measuring guide to ensure a proper fit. Filter Size 17. 75" x 27. 75" x 1" Premium Airflow Efficiency High performance cube core eggcrate design allows 95% airflow efficiency, reducing energy costs and maximizing HVAC system lifespan. Durable, Rust Free Aluminum